Refer to the exhibit.

For traffic that does not match any of the defined SD-WAN rules, the default implicit SD-WAN rule is applied. By default, the FortiGate uses a "source-destination IP-based" algorithm, which means all traffic from a specific source IP to a specific destination IP is sent through the same interface. This ensures that a consistent path is used for traffic between the same source and destination IP addresses. Options B, C, and D do not apply because the default algorithm does not prioritize by latency, session count, or source IP alone.

References:

FortiOS 7.4.1 Administration Guide: SD-WAN Load Balancing Algorithms​